Is velcro use acceptable in explosive environments? 3

[zappedagain]

A colleague wants to use Velcro to attach cables to a robot arm in a paint booth that is classified as Class I Division 1. The only threads I see about this are from NASA in the 1970s. I see some people make anti-static Velcro straps for ESD controlled environments. I'm sure they work, but are they needed?
Thanks,

Z

 

[ScottyUK]

I told my wife that if our site ever has a Very Bad Day then our kids will be able to play on the beach every day after school, because the beach will suddenly be at the end of our street. We live 8 miles from the sea... I'm not sure she gets my humour.

 

[MikeHalloran]

Yeah, I worked at a plant like that once. We made pharmaceuticals, but one of the intermediates is explosive. The plant has an aggressive safety program, and its own fire brigade, but everybody knows a really bad day will end with a really big smokin' hole in the ground.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

I had that sort of situation back during the Cold War. I worked across the street from a naval weapons depot that purportedly stored nuclear weapons. Sort of machts nichts, given that we were a defense contractor, so the bomb was either going to hit us or hit across the street. Either way, we were going to be toast. Not even enough time to duck and cover, as if...

TTFN
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[LiteYear]

My gut feel is that the energy released in a spark generated by a velcro rip is too small to be a problem. The electrostatic charge is a different problem - our requirements (Group I, Zone 0, AusEx/IECEx) restrict the continuous region of non-conductive plastic to be less than 100mm2. Any more than that could store a hazardous amount of energy and needs to be made conductive. So as long as your velcro is small enough, ESD is not a problem. But check your standard - Group II can be a lot more stringent (eg. 525uJ is magic number for methane, but from memory it's as low as 20uJ for ethylene!).

The only way to be sure would be to mechanically rip apart some sample velcro in a chamber with your hazardous dust. We do the equivalent exercise in methane chambers regularly whenever we have a new material/design/arrangement and need to get it Ex approved. If it goes bang, it's a no go.

For those incredulous at the requirements of hazardous area certification, be assured it is real and on-going. Contraband is a serious issue in underground mines in Australia - no aluminium, no watches, no phones, nothing that could release enough energy to ignite methane. It's hard to believe that people could work in such hazardous environments, where the mere presence of aluminium is a concern, but it works. Lots and lots of study goes into setting the limits and manufacturers manage (through quite extreme constraint) to comply. The problem with aluminium in an underground mine is that things are always bumping into other things, including mobile machines into boring old enclosures. Aluminium is known to be a problem, so we do without it.

Which reminds me - another avenue of investigation might be your customer - if they have a site rule that says employees can't wear velcro strapped watches then there's your answer.

 

[zappedagain]

Thanks. The install is occurring next week, so I expect I'll get some feedback from the safety engineer on site.

Scotty, that's some nice history you dug up there. Kudos to you!

Z

 

[ScottyUK]

Thank you.

 

[ornerynorsk]

It would be well worth a look at existing loom and raceway clips that are available as stock items.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.